A circuit board manufacturing process typically involves creating a bare printed circuit board or PCB (i.e., circuit board fabrication), and subsequently populating that circuit board with components (i.e., circuit board assembly). Prior to surface mount technology (SMT), circuit board manufacturers made circuit boards predominantly using pin-in-hole technology. To populate a circuit board using pin-in-hole technology, a manufacturer typically solders components having wire-leads to the circuit board using a wave soldering process. In particular, the manufacturer inserts wire-leads of the components through copper vias or plated through holes (PTHs) within the circuit board from a top side of the circuit board. The manufacturer then passes the circuit board horizontally through a solder wave machine that provides a standing wave of molten solder. As the circuit board passes through the wave of molten solder, solder flows into the copper vias thus soldering the wire-leads of the components to the copper vias of the circuit board.
At the advent of surface mount technology (SMT), manufacturers created hybrid circuit boards that supported both SMT devices and pin-in-hole devices for use with solder wave machinery. Such hybrid PCBs include square or rectangular circuit board pads (e.g., Hot Air Solder Level or HASL pads) for SMT devices, and copper vias for wire-leaded devices. To form square or rectangular SMT circuit board pads (hereinafter generally referred to as simply rectangular circuit board pads), the circuit board manufacturer forms the pads using either a copper defined or solder mask defined process. In one version of the copper defined process, the manufacturer disposes a mask layer (i.e., a thin light-sensitive film) over an outer conductive layer (copper clad) of the circuit board. The manufacturer exposes portions of the mask layer to light and develops the mask layer so that what remains of the mask layer is network of masking portions, i.e., masking structures that define traces, pads, etc. In particular, the masking portions define substantially straight sides and crisp, sharp 90 degree angled corners for the rectangular circuit board pads. The manufacturer can perform these steps for the other side of the circuit board as well. The manufacturer then puts the circuit board through an etching bath which etches away unprotected portions of the outer conductive layer leaving conductive material under the network of masking portions. The corners of the rectangular circuit board pads resulting from the etching process may have up to a 3 mil radius due to etching activity at the corners where there is more exposure to the etchant than at the substantially straight sides. The manufacturer then removes the remaining masking portions leaving a clean circuit board which is ready for assembly.
It should be noted that there are many alternate process variations used to manufacture the external copper features of a circuit board. The above-described subtractive version for making copper defined pads is just one variation. One will appreciate that additive circuits may be produced where the described copper features are additively plated up over bare laminate. There are subtractive-additive processes as well, i.e., processes that employ light sensitive film and pattern plating (e.g., developing the film and subsequently plating over initial copper conductive pads and traces) with a metal layer that will protect the copper features during etching. One will appreciate, therefore, that the earlier-described light sensitive film version is provided simply to illustrate just one generic version out of many for creating copper traces and solderable surfaces in this discussion.
In the solder mask defined process, the manufacturer fabricates a hybrid circuit board having rectangular pads with sharp 90 angled corners. Over the fabricated hybrid circuit board, the manufacturer applies a final solder mask layer which also defines rectangular pad apertures with sharp 90 angled corners which mirror the shapes of the pads. This solder mask layer slightly overlaps the copper of the pads (e.g., by several mils) so that the outer shape of the solder joints that eventually form over the pads is essentially determined by the shape of the rectangular pad apertures defined by the solder mask.
To populate a hybrid circuit board having either copper defined or solder mask defined pads using solder wave machinery, the manufacturer glues the surface mount devices to the circuit board such that contacts of the surface mount devices reside directly over the rectangular circuit board pads. Additionally, the manufacturer inserts wire-leads of the wire-lead devices through the copper vias, as explained earlier. The manufacturer then passes the hybrid circuit board through the solder wave machine. As the hybrid circuit board passes through the wave of molten solder, solder clings to both the rectangular circuit board pads and the contacts of the SMT devices, as well as flows into the copper vias. As a result, solder joints form between the rectangular circuit board pads and the contacts of the SMT devices. Solder joints also form between the copper vias of the circuit board and the wire-leads of the wire-lead devices, as described earlier.
Today, many circuit board manufacturers have replaced their solder wave machinery with SMT equipment that utilizes a solder printing process. Here, the manufacturer prints solder paste over the rectangular circuit board pads of the circuit board (e.g., using a stencil), and then positions surface mount components over the rectangular circuit board pads and in contact with the printed solder paste (e.g., using automated pick-and-place equipment). The manufacturer then passes the circuit board through an oven that applies heat to activate flux and to melt solder within the solder paste. As a result, the flux strips the pad surfaces of oxidation and the solder liquefies and wets to the pad surfaces to form solder joints between the rectangular circuit board pads and the contacts of the SMT devices.
Some manufacturers have made enhancements to their SMT printed solder processes. For example, some manufacturers provide round or hexagonal circuit board pads rather than square or rectangular circuit board pads for certain SMT devices (e.g., for Ball Grid Array components having a high density of contacts). As another example, some manufacturers provide rectangular-shaped pads with slightly rounded corners for soldering to SMT devices having gull-wing leads. It is believed that these manufacturers provide the rounded corners to these discreet pads because it is easier for these manufacturers to control the etching tolerances of such pads vis-à-vis circuit board pads with sharp 90 degree corners. As yet another example, some manufacturers provide pads having the shape of a semi-circle for discreet components.